Laser-induced incandescence (LII) is an analytical technique that can be used for in-situ measurements of soot or other particles in environments such as flames, combustion chambers, the atmosphere, etc. Unlike most laser-based gas-phase diagnostic techniques, LII is a technique that is not performed along the line-of-sight of the laser and is not typically strongly dependent on laser wavelength. Rather, LII is directed toward heating particulates suspended in a gas phase to incandescence, thereby stimulating emission of blackbody or quasi-blackbody radiation. Because the thermal radiation is incoherent and emits with little or no preference for direction, its measurement is typically performed off-axis with respect to the laser propagation, and the amount of incandescent radiation received by an LII detector will increase proportionally as the solid angle of the region observed by the detector is increased.
The blackbody or quasi-blackbody spectrum of the incandescent radiation will vary depending on the temperature that the particles reach and their composition, and the total emission will be strongly dependent on the temperature that the particles reach. Measurements of the spectrum of the incandescence allow the temperature of the particles to be estimated. As particles are heated to their vaporization temperature, a maximum temperature will be reached that is dependent on the material properties of the particles. Levels of emission of incandescent radiation by the particles over time as the particles heat depends on material and optical properties of the particles. The amount of LII scales with the volume of material in the particles. The heating rate of particles during LII is proportional to the absorption of the particles at the excitation wavelength, and the absorption spectrum of the particles is influenced by the material composition and optical properties. Properties of an analyte specimen such as spatial distribution, concentration, absorption coefficient, and composition can be determined based upon analysis of the radiation emitted as a result of the incandescence of the analyte.
LII relies on the extremely high spatial coherence of laser radiation to generate the high optical intensities required to heat particles to incandescence. A key parameter of the laser is its fluence, a measure of energy delivered per unit area. Unlike most laser-based gas-phased diagnostic techniques, LII has a non-linear dependence on the laser energy. For a given particulate analyte specimen, a certain threshold fluence of the laser must be reached for the laser to impart enough energy to the particulates to bring them to their vaporization point and produce incandescence at the particle vaporization temperature.
Information that can be gleaned from LII measurements of a specimen can depend in part on a level of incandescence signal that can be stimulated in the specimen by the laser. In general, this signal level is higher when 1) a density of the particulate analyte is higher or 2) when a larger area is illuminated by the laser while maintaining a same level of fluence. The density of the particulate analyte is sample-dependent, and is generally a value desirably measured rather than a parameter to be controlled. Thus, when the density of the analyte is relatively low, a diameter of the laser beam used to illuminate the sample can be increased to cause a greater level of signal incandescence. In order to maintain a required threshold fluence for a larger beam diameter at a given pulse repetition rate, however, the power output of the laser must be increased. In some cases, increasing the power output of the laser is prohibitive, as the power output required to generate a desired signal level is too high to practically realize. In other cases, a sufficiently high-power laser may be built, but it may be so large or unwieldy as to be unsuitable for in-situ field measurements of samples.